Time-of-flight depth mapping with parallax compensation

ABSTRACT

An optical sensing device includes a light source, which is configured to emit one or more beams of light pulses at respective angles toward a target scene. An array of sensing elements is configured to output signals in response to incidence of photons on the sensing elements. Light collection optics are configured to image the target scene onto the array. Control circuitry is coupled to actuate the sensing elements only in one or more selected regions of the array, each selected region containing a respective set of the sensing elements in a part of the array onto which the light collection optics image a corresponding area of the target scene that is illuminated by the one of the beams, and to adjust a membership of the respective set responsively to a distance of the corresponding area from the device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/950,186, filed Apr. 11, 2018, which claims the benefit of U.S.Provisional Patent Application 62/526,375, filed Jun. 29, 2017, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to depth mapping, andparticularly to devices and methods for depth mapping based on sensingof time of flight (ToF).

BACKGROUND

Time-of-flight (ToF) imaging techniques are used in many depth mappingsystems (also referred to as 3D mapping or 3D imaging systems). Indirect ToF techniques, a light source, such as a pulsed laser, directspulses of optical radiation toward the scene that is to be mapped, and ahigh-speed detector senses the time of arrival of the radiationreflected from the scene. (The terms “light” and “illumination,” as usedin the context of the present description and in the claims, refer tooptical radiation in any or all of the visible, infrared and ultravioletranges.) The depth value at each pixel in the depth map is derived fromthe difference between the emission time of the outgoing pulse and thearrival time of the reflected radiation from the corresponding point inthe scene, which is referred to as the “time of flight” of the opticalpulses. The radiation pulses that are reflected back and received by thedetector are also referred to as “echoes.”

Some ToF-based depth mapping systems use detectors based onsingle-photon avalanche diode (SPAD) arrays. SPADs, also known asGeiger-mode avalanche photodiodes (GAPDs), are detectors capable ofcapturing individual photons with very high time-of-arrival resolution,of the order of a few tens of picoseconds. They may be fabricated indedicated semiconductor processes or in standard CMOS technologies.Arrays of SPAD sensors, fabricated on a single chip, have been usedexperimentally in 3D imaging cameras. Charbon et al. provide a review ofSPAD technologies in “SPAD-Based Sensors,” published in TOFRange-Imaging Cameras (Springer-Verlag, 2013).

For efficient detection, SPAD arrays may be integrated with dedicatedprocessing circuits. For example, U.S. Patent Application Publication2017/0052065, whose disclosure is incorporated herein by reference,describes a sensing device that includes a first array of sensingelements (such as SPADs), which output a signal indicative of a time ofincidence of a single photon on the sensing element. A second array ofprocessing circuits are coupled respectively to the sensing elements andcomprise a gating generator, which variably sets a start time of thegating interval for each sensing element within each acquisition period,and a memory, which records the time of incidence of the single photonon each sensing element in each acquisition period. A controllerprocesses a histogram of respective counts over different time bins foreach sensing element so as to derive and output a respectivetime-of-arrival value for the sensing element.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide improved devices and methods for ToF-based depth mapping.

There is therefore provided, in accordance with an embodiment of theinvention, an optical sensing device, including a light source, which isconfigured to emit one or more beams of light pulses at respectiveangles toward a target scene. An array of sensing elements is configuredto output signals in response to incidence of photons on the sensingelements. Light collection optics are configured to image the targetscene onto the array. Control circuitry is coupled to actuate thesensing elements only in one or more selected regions of the array, eachselected region containing a respective set of the sensing elements in apart of the array onto which the light collection optics image acorresponding area of the target scene that is illuminated by the one ofthe beams, and to adjust a membership of the respective set responsivelyto a distance of the corresponding area from the device.

In the disclosed embodiments, the signals output by the sensing elementsare indicative of respective times of arrival of the photons on thesensing elements, and the control circuitry is configured to process thesignals in order to compute an indication of the distance to thecorresponding area in the target scene based on the times of arrival. Inone embodiment, the sensing elements include single-photon avalanchediodes (SPADs). Additionally or alternatively, the control circuitry isconfigured to bin together the signals that are output by the sensingelements in the set in order to compute an average time of flight of thephotons over the set.

In a disclose embodiment, the light source includes a plurality ofemitters, which are configured to emit a corresponding plurality of thebeams concurrently toward different, respective areas of the targetscene.

In one embodiment, the control circuitry is configured to enlarge theselected region of the array responsively to the distance, such that theselected region contains a larger number of the sensing elements whenthe corresponding area is close to the device than when thecorresponding area is far from the device.

Additionally or alternatively, the device is configured to sense thephotons received from the target scene over a range of distances from aminimal range to a maximal range, and the control circuitry isconfigured to set a size of the selected region to be sufficient tocontain a first image cast onto the array by the light collection opticsof the corresponding area of the scene at the maximal range, but smallerthan a second image cast onto the array by the light collection opticsof the corresponding area of the scene at the minimal range.

There is also provided, in accordance with an embodiment of theinvention, an optical sensing device, including a light source, which isconfigured to emit one or more beams of light pulses along a transmitaxis toward a target scene. An array of sensing elements is configuredto output signals in response to incidence of photons on the sensingelements. Light collection optics are configured to image the targetscene onto the array along a receive axis, which is offset transverselyrelative to the transmit axis. Control circuitry is coupled to actuatethe sensing elements only in one or more selected regions of the array,each selected region containing a respective set of the sensing elementsin a part of the array onto which the light collection optics image acorresponding area of the target scene that is illuminated by the one ofthe beams, while setting a boundary of the selected region responsivelyto a parallax due to the offset between the transmit and receive axes.

In a disclosed embodiment, the control circuitry is configured to shifta boundary of the selected region of the array as responsively to adistance of the corresponding area from the device in order tocompensate for the parallax.

Additionally or alternatively, the device is configured to sense thephotons received from the target scene over a range of distances from aminimal range to a maximal range, such that a first image cast onto thearray by the light collection optics of the corresponding area of thescene at the maximal range is shifted transversely, due to the parallax,relative to a second image cast onto the array by the light collectionoptics of the corresponding area of the scene at the minimal range. Thecontrol circuitry is configured to set the boundary of the selectedregion to contain all of the first image but only a part of the secondimage.

There is additionally provided, in accordance with an embodiment of theinvention, a method for optical sensing, which includes emitting one ormore beams of light pulses at respective angles toward a target scene.The target scene is imaged onto an array of sensing elements configuredto output signals in response to incidence of photons on the sensingelements. The sensing elements are actuated only in one or more selectedregions of the array, each selected region containing a respective setof the sensing elements in a part of the array onto which acorresponding area of the target scene that is illuminated by the one ofthe beams is imaged. A membership of the respective set is adjustedresponsively to a distance of the corresponding area from the array.

In some embodiments, the one or more beams are emitted along a transmitaxis, while the target scene is imaged onto the array along a receiveaxis, which is offset transversely relative to the transmit axis, andadjusting the membership includes changing the membership so as tocompensate for a parallax between the transmit and receive axes as afunction of the distance.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a depth mapping device, in accordancewith an embodiment of the invention;

FIG. 2 is a block diagram that schematically illustrates a ToF detectionarray, in accordance with an embodiment of the invention;

FIG. 3 is a schematic frontal view of reflected laser spots imaged onlyelements of a SPAD array, in accordance with an embodiment of theinvention;

FIGS. 4A-C are schematic frontal views of a sequence of super-pixelsdefined in operation of a ToF detection array, in accordance with anembodiment of the invention; and

FIGS. 5A-C are schematic frontal views of a sequence of super-pixelsdefined in operation of a ToF detection array, in accordance withanother embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

U.S. Patent Application Publication 2017/0176579, whose disclosure isincorporated herein by reference, describes a ToF-based depth mappingsystem in which echoes of transmitted laser pulses are sensed by atwo-dimensional array of single-photon time-sensitive sensing elements,such as SPADs. By addressing each SPAD individually via dedicatedcontrol circuitry, the sensitivity, including the on/off-state, of eachSPAD is controlled by its specific reverse p-n junction high voltage. Atany instant during a scan, only the sensing elements in the area orareas of the array that are to receive reflected illumination from anemitted beam are actuated. The sensing elements are thus actuated onlywhen their signals provide useful information. This approach bothreduces the background signal, thus enhancing the signal-to-backgroundratio, and lowers the electrical power needs of the detector array.

In some embodiments, several SPADs are grouped together into“super-pixels,” meaning that these SPADs are actuated at the same time,and the pulses that they generate due to incident photons are binnedtogether for purposes of ToF measurement. The super-pixels that areactuated at any given time are those on which reflected photons from agiven transmitted pulse are expected to be incident. Thus, if thetransmitted and reflected beams are tightly focused, the reflected pulsewill be incident on only a single SPAD or a small group of SPADs, andthe super-pixels need comprise only a few adjacent SPADs. On the otherhand, when the reflected beam extends over a larger area of the SPADarray, it is advantageous to use larger super-pixels.

If the beam paths of the transmitted and reflected pulses are coaxial, apulse transmitted in a given angular direction from the ToF-basedmapping device will always be reflected back to the same SPAD or groupof SPADs in the array, regardless of the distance to the object in thescene from which the pulse is reflected. Such a coaxial arrangementgenerally dictates the use of a beam combiner, which leads to loss ofsignal and imposes other design constraints. Therefore, in theembodiments described hereinbelow, the transmitter and the receiver arepositioned side by side with an offset between their respective opticalaxes.

The offset between the transmitter and receiver, however, gives rise toissues of parallax: The SPAD or group of SPADs that receive thereflection of a pulse transmitted in a given angular direction will varyas a function of the distance to the area in the scene on which thepulse is incident. Furthermore, in both non-coaxial and coaxialconfigurations, the size of the reflected spot on the SPAD array willtypically change with distance due to defocusing, and specifically maybe larger for nearby objects than for distant objects. To accommodatethese distance-dependent changes in the location and size of thereflected spot, it is possible to use large super-pixels, which willcapture substantially all reflected photons regardless of the distanceto the object. This approach, however, increases both the powerconsumption and the noise generated in the SPAD array, since asubstantially larger number of SPADs are actuated than are actuallyneeded to receive all the reflected photons from the object at any givendistance. This problem is particularly acute for distant objects, fromwhich the reflected light signals are relatively weak.

Embodiments of the present invention that are described herein addressthis problem by providing dynamic super-pixels, which change theirconfiguration depending on distance to the target scene. Thus, thelocation of the super-pixel that is to receive reflected light from abeam transmitted at a given angle is selected, and can shift, so as toaccommodate the effect of parallax as a function of object distance.Additionally or alternatively, the size of the super-pixel is set, andmay increase or decrease, in order to handle changes in the reflectedspot size.

The disclosed embodiments provide an optical sensing device, whichcomprises a light source and an array of sensing elements. The lightsource emits one or more beams of light pulses at respective anglestoward a target scene, and light collection optics image the targetscene onto the array of sensing elements, which output signals inresponse to incident photons. In the embodiments described below, thelight source comprises an array of emitters, but the principles of thepresent invention may alternatively be applied, mutatis mutandis, toscanned beams, as in the above-mentioned U.S. Patent ApplicationPublication 2017/0176579.

Control circuitry actuates the sensing elements only in one or moreselected regions of the array, which are defined as super-pixels. Eachsuch super-pixel contains a respective set of the sensing elements in apart of the array onto which the light collection optics image acorresponding area of the target scene that is illuminated by one of theemitted beams. The control circuitry adjusts the membership of the setof sensing elements depending upon the distance of the correspondingarea from the device. In some embodiments, these adjustments includeenlarging and shrinking the selected region of the array, such that thesuper-pixel may contain a larger number of the sensing elements when thecorresponding area is close to the device than when the correspondingarea is far from the device.

Additionally or alternatively, in some embodiments, the super-pixelboundaries are set and may be shifted to take parallax into account, dueto the transverse offset of the receive axis of the light collectionoptics relative to the transmit axis of the light source. (As notedabove, the offset between the transmit and receive axes causes the imageof the area of the scene corresponding to a given super-pixel to shifttransversely with the distance of the area from the device.) In someembodiments, the control circuitry shifts the boundaries of thesuper-pixels in order to compensate for this parallax, for example bysetting and shifting the boundaries of the super-pixel depending on thedistance of the corresponding area from the device. In otherembodiments, the boundaries of the super-pixels are set so as to containthe image cast onto the array when the area corresponding to thesuper-pixel is at the maximal range from the device, while containingonly a part of the image cast onto the array when the corresponding areaof the scene is at the minimal range (where it is expected that thesignals output by the array will be stronger anyway).

System Description

FIG. 1 is a schematic side view of a depth mapping device 20, inaccordance with an embodiment of the invention. Device 20 comprises atransmitting (Tx) laser projector 22 and a receiving (Rx) camera 24,with respective optical axes 26 and 28 that are offset transversely by abaseline offset B, as shown in the figure.

Tx laser projector 22 comprises an array 30 of emitters, such as amonolithic array of vertical-cavity surface-emitting lasers (VCSELs),which concurrently emit respective beams of light pulses. Collimatingoptics 32 project these beams at different, respective angles, towardcorresponding areas of a target scene. To increase the number ofprojected beams in the pictured embodiment, a diffractive opticalelement (DOE) 34 splits the projected beam pattern into multipleadjacent or overlapping copies, thus creating a denser pattern of spotsextending over the target scene. A cover window 36 of the deviceincludes a filter 38, for example an infrared (IR) filter, in order toprevent light outside the optical working range from exiting andentering the device.

Rx camera 24 comprises an array 40 of sensing elements, which outputsignals in response to incident photons. In the present embodiment, thesensing elements comprise SPADs, or possibly another type ofsingle-photon detector, so that the output signals are indicative ofrespective times of arrival of photons on the sensing elements. Lightcollection optics 42 image the target scene onto the SPAD array, while abandpass filter 44 blocks incoming light that is outside the emissionband of the Tx laser projector.

Each of the beams emitted by Tx laser projector 22 illuminates acorresponding area of the target scene, and light collection optics 42image this area onto a certain, respective region of SPAD array 40.Control circuitry (shown in FIG. 2 ) actuates respective sets of theSPADs only in these particular regions, as explained above, and in somecircumstances selects the membership of these sets depending upon thedistance of the corresponding area of the target scene from the depthmapping device. These distances typically vary over a certain designrange, from a predefined minimum to a predefined maximum, over which thedevice is expected to be able to make depth measurements.

The selection of member SPADs can take into account, inter alia, theparallax due to the offset between Tx and Rx axes 26 and 28. Asexplained earlier, this parallax causes the region of SPAD array 40 ontowhich the spot due to a given laser beam is imaged to shift transverselyin dependence upon the distance to the area of the target scene that isilluminated by the beam. This parallax shift is illustrated, forexample, by the beams that are reflected from a near object 46 and adistant object 48 in FIG. 1 . Details of some methods and criteria thatcan be applied in selecting the SPADs to actuate are presented withreference to the figures that follow.

Although the embodiments shown in the figures and described herein referto the particular design of depth mapping device 20, the principles ofthe present invention may similarly be applied, mutatis mutandis, toother sorts of optical sensing devices that use an array of sensingelements, for both depth mapping and other applications. For example, Txlaser projector 22 may comprise a scanner, which scans a single beam oran array of multiple beams over the target scene. As another example, Rxcamera 24 may contain detectors of other sorts, which may detectreflected light intensity in addition to or instead of time of flight.Furthermore, some of the distance-based adjustments of super-pixelboundaries that are described herein are also applicable to devices inwhich the Tx laser projector and the Rx camera are coaxial (with asuitable beam combiner, for example). All such alternative embodimentsare considered to be within the scope of the present invention.

FIG. 2 is a block diagram that schematically shows details of Rx camera24, in accordance with an embodiment of the invention. In thisembodiment, SPAD array 40 includes integrated addressing logic 50, aswell as processing circuits 52. Circuits of this sort are described, forexample, in the above-mentioned U.S. Patent Application Publications2017/0052065 and 2017/0176579, as well as in U.S. Patent ApplicationPublication 2017/0179173, whose disclosure is also incorporated hereinby reference.

SPAD array 40 in the pictured embodiment comprises an addressable matrixof 170×130 SPADs 54. Control circuitry 56 interacts with X- andY-addressing logic 50 to select the SPADs 54 that are to be actuated atany given time. The selected SPADs are turned on, for example by settingthe bias voltages appropriately, so as to emit pulses in response toincident photons, while the remaining SPADs are deactivated. The pulsesare amplified and shaped by an analog front end (AFE) 58, operating in anumber of parallel channels (135 channels in the present example, eachserving a super-pixel comprising four active SPADs). Selection logic 60connects the AFE channels to respective time-to-digital converters (TDC)62, which output digital values indicative of the times of arrival ofthe pulses, synchronized by a phase-locked loop (PLL) 64.

A histogram builder 66 collects a histogram 68 of the digital valuesfrom each TDC channel over a series of transmitted pulses. Based onthese histograms, a readout (R/O) circuit 70 outputs a ToF value foreach super-pixel (given by the mode of the histogram, for example), thusdefining a depth map of the target scene. Alternatively or additionally,processing circuits 52 may fully output raw histograms for eachsuper-pixel for further processing in the control circuitry, orelsewhere in the system. When multiple SPADs 54 are binned together intoa super-pixel, the ToF of each detected photon from any SPAD in thesuper-pixel feeds into a common histogram builder circuit. The ToF valuein this case will represent an average time of flight of the signalphotons over the SPADs in this set. The ToF values, as well as otherfeatures of the signals output by the active SPADs, may be fed back tocontrol circuitry 56 for use in subsequent adjustments.

Control circuitry 56 selects the set of SPADs 54 to actuate in eachsuper-pixel 72 and adaptively adjusts this set, thus setting andshifting the super-pixel boundaries. Collection optics 42 in Rx camera24 image the area illuminated by each transmitted laser spot onto acertain, respective region 74 of SPAD array 40, as illustrated in FIG. 2. (It is assumed in this figure that the areas illuminated by the spotsare at equal, relatively large distances from depth mapping device 20.)Thus, in FIG. 2 , the active SPADs 54 selected by control circuitry 56are those onto which the laser spots are imaged, so that eachsuper-pixel 72 includes a set of 2×2 SPADs. Control circuitry 56 is ableto adjust the super-pixel boundaries in response to the respectivedistances of the illuminated areas from the depth mapping device,meaning that in practice, at any given point in time, some of thesuper-pixels may be larger than others and/or may have their boundariesshifted transversely relative to the situation shown in FIG. 2 .

Super-Pixel Selection and Control

FIG. 3 is a schematic frontal view of reflected laser spots 80, 82, 84,86, 88, 90 imaged from an area of a target scene onto SPADs 54 in array40, in accordance with an embodiment of the invention. This figuresshows the effects of parallax and defocus on the size and location ofthe region of the SPAD array onto which the spot produced by a givenlaser beam is imaged by collection optics 42, as a function of thedistance of the area of the target scene illuminated by the laser beamfrom the depth mapping device. It assumes a certain baseline offset (B)between Tx and Rx axes 26 and 28 (FIG. 1 ). The “pixels” in the figurecorrespond to individual SPADs 54 in the array. The distance to thetarget scene can vary over a range up to two orders of magnitude.

Spot 80, at the left side of FIG. 3 , represents the image formed onSPAD array 40 when the laser beam is incident on an area of the targetscene 5 m away from depth mapping device 20; whereas spot 90 at theright side represents the image when the beam is incident on an areaonly 15 cm away. Spot 80 covers a region roughly two pixels in diameter(D₁), while spot 90 covers a region almost three pixels in diameter(D₂), due to defocus (blurring) of the image by collection optics 42. Atthe same time, the center of spot 90 has shifted relative to spot 80 byabout 2.5 pixels due to parallax. These specific results are a functionof the optical properties and geometrical dimensions of a specific depthmapping device, but the principles of defocus and shift will apply toother depth mapping devices of similar configuration.

To deal with the defocus effect, control circuitry 56 may enlarge thesuper-pixels that receive light from areas of the target scene at shortdistances. Thus each such super-pixel will contain a larger number ofSPADs 54 when the corresponding area of the target scene is close todepth mapping device 20 than when the corresponding area is far from thedevice.

Alternatively, the relatively small size of super-pixels 72 may bemaintained regardless of distance, so that the size of the super-pixelis sufficient to contain the image of the illumination spot that is castonto the array by the collection optics from distant objects (forexample, 2×2 pixels) but smaller than the larger (defocused) image thatis cast onto the array from nearby objects. Because nearby objectsgenerally return a much larger flux of photons from the laser beam backonto SPAD array 40, the use of the smaller super-pixel will still giveadequate signals at short range, while minimizing the background noisein the weaker signals collected at long range. On this basis, controlcircuitry 56 may even set the super-pixel boundaries statically, withoutactively accommodating for parallax effects, so as to contain all of theimage of the illumination spot cast from distant objects whilecontaining only a part of the image cast from nearby objects.

FIGS. 4A-C are schematic frontal views of a sequence of super-pixels 92,94, 96 defined in operation of SPAD array 40, in accordance with anembodiment of the invention. These figures, as well as FIGS. 5A-C,exemplify methods by which control circuitry 56 shifts the super-pixelboundary dynamically in order to compensate for the shift of the imageof the laser spot with distance to the corresponding area of the targetscene, due to the parallax effect explained above. In this example, thelocation of the super-pixel shifts as a function of the distance fromsuper-pixel 92 (FIG. 4A) for nearby areas, through super-pixel 94 at anintermediate distance (FIG. 4B), to super-pixel 96 for distant objects(FIG. 4C).

Since the shift in spot position due to parallax is deterministic as afunction of range, control circuitry 56 can reliably change thesuper-pixel boundary as a function of the measured ToF. The boundary ofeach super-pixel can be incremented in a number of discrete steps, asillustrated in FIGS. 4A-C, to the location that gives the best signalfor the super-pixel in question. Thus, for example, super-pixel 92 maybe used for ToF<2 ns (targets closer than 30 cm); super-pixel 94 for 2ns<ToF<10 ns (targets between 30 cm and 1.5 m from device 20); andsuper-pixel 96 for ToF>10 ns (targets at 1.5 m and beyond).

FIGS. 5A-C are schematic frontal views of a sequence of super-pixels100, 102, 104 defined in operation of SPAD array 40, in accordance withanother embodiment of the invention. In this embodiment, controlcircuitry 56 initially sets the super-pixel boundary to the large sizeof super-pixel 100, which is capable of capturing photons from bothnearby and distant areas of the target scene, as illustrated in FIG. 5A.As the target distance increases, control circuitry 56 shifts one edgeof the super-pixel boundary (the right edge in the pictured example) tothe intermediate size of super-pixel 102, shown in FIG. 5B, so as toreduce background due to unneeded SPADs 54 while still capturing all ofthe image of the illumination spot. Super-pixel 104 is reduced tominimal size for areas of the target scene that are found to be at largedistances from depth mapping device 20, as shown in FIG. 5C.

Although the foregoing figures show certain particular schemes fordynamic adjustment of super-pixel boundaries, alternative schemes willbe apparent to those skilled in the art after reading the abovedescription and are considered to be within the scope of the presentinvention. It will thus be appreciated that the embodiments describedabove are cited by way of example, and that the present invention is notlimited to what has been particularly shown and described hereinabove.Rather, the scope of the present invention includes both combinationsand subcombinations of the various features described hereinabove, aswell as variations and modifications thereof which would occur topersons skilled in the art upon reading the foregoing description andwhich are not disclosed in the prior art.

The invention claimed is:
 1. An optical sensing device, comprising: alight source, which is configured to emit one or more beams of lightpulses at respective angles toward a target scene; an array of sensingelements configured to output signals in response to incidence ofphotons on the sensing elements; light collection optics configured toimage the target scene onto the array; and control circuitry coupled toactuate the sensing elements only in one or more selected regions of thearray, each selected region containing a respective set of the sensingelements in a part of the array onto which the light collection opticsimage a corresponding area of the target scene that is illuminated bythe one of the beams, and to adjust a membership of the respective setresponsively to a distance of the corresponding area from the device,wherein each selected region has a boundary comprising multiple edges,which contain the respective set of the sensing elements, and whereinthe control circuitry is configured to enlarge the selected region ofthe array responsively to the distance by shifting one edge of theboundary, such that the selected region contains a larger number of thesensing elements when the corresponding area is close to the device thanwhen the corresponding area is far from the device.
 2. The deviceaccording to claim 1, wherein the signals output by the sensing elementsare indicative of respective times of arrival of the photons on thesensing elements, and wherein the control circuitry is configured toprocess the signals in order to compute an indication of the distance tothe corresponding area in the target scene based on the times ofarrival.
 3. The device according to claim 2, wherein the sensingelements comprise single-photon avalanche diodes (SPADs).
 4. The deviceaccording to claim 2, wherein the control circuitry is configured to bintogether the signals that are output by the sensing elements in the setin order to compute an average time of flight of the photons over theset.
 5. The device according to claim 1, wherein the light sourcecomprises a plurality of emitters, which are configured to emit acorresponding plurality of the beams concurrently toward different,respective areas of the target scene.
 6. The device according to claim1, wherein the light source emits the one or more beams along a transmitaxis, while the light collection optics image the target scene onto thearray along a receive axis, which is offset transversely relative to thetransmit axis, and wherein the control circuitry is configured to adjustthe membership so as to compensate for a parallax between the transmitand receive axes as a function of the distance.
 7. The device accordingto claim 6, wherein the control circuitry is configured to shift theboundary of each selected region of the array responsively to thedistance in order to compensate for the parallax.
 8. The deviceaccording to claim 1, wherein the device is configured to sense thephotons received from the target scene over a range of distances from aminimal range to a maximal range, and wherein the control circuitry isconfigured to set a size of the one or more selected regions to besufficient to contain a first image cast onto the array by the lightcollection optics of the corresponding area of the scene at the maximalrange, but smaller than a second image cast onto the array by the lightcollection optics of the corresponding area of the scene at the minimalrange.
 9. An optical sensing device, comprising: a light source, whichis configured to emit one or more beams of light pulses along a transmitaxis toward a target scene; an array of sensing elements configured tooutput signals in response to incidence of photons on the sensingelements; light collection optics configured to image the target sceneonto the array along a receive axis, which is offset transverselyrelative to the transmit axis; and control circuitry coupled to actuatethe sensing elements only in one or more selected regions of the array,each selected region containing a respective set of the sensing elementsin a part of the array onto which the light collection optics image acorresponding area of the target scene that is illuminated by the one ofthe beams, while setting a boundary of each selected region responsivelyto a parallax due to the offset between the transmit and receive axes,wherein the boundary of each selected region comprises multiple edges,which contain the respective set of the sensing elements, and whereinthe control circuitry is configured to enlarge the selected region ofthe array responsively to the parallax by shifting one edge of theboundary, such that the selected region contains a larger number of thesensing elements when the corresponding area is close to the device thanwhen the corresponding area is far from the device.
 10. The deviceaccording to claim 9, wherein the device is configured to sense thephotons received from the target scene over a range of distances from aminimal range to a maximal range, such that a first image cast onto thearray by the light collection optics of the corresponding area of thescene at the maximal range is shifted transversely, due to the parallax,relative to a second image cast onto the array by the light collectionoptics of the corresponding area of the scene at the minimal range, andwherein the control circuitry is configured to set the boundary of theselected region to contain all of the first image but only a part of thesecond image.
 11. A method for optical sensing, comprising: emitting oneor more beams of light pulses at respective angles toward a targetscene; imaging the target scene onto an array of sensing elementsconfigured to output signals in response to incidence of photons on thesensing elements; actuating the sensing elements only in one or moreselected regions of the array, each selected region containing arespective set of the sensing elements in a part of the array onto whicha corresponding area of the target scene that is illuminated by the oneof the beams is imaged; and adjusting a membership of the respective setresponsively to a distance of the corresponding area from the arraywherein each selected region has a boundary comprising multiple edges,which contain the respective set of the sensing elements, and whereinadjusting the membership comprises enlarging the selected region of thearray responsively to the distance by shifting one edge of the boundary,such that the selected region contains a larger number of the sensingelements when the corresponding area is close to the array than when thecorresponding area is far from the array.
 12. The method according toclaim 11, wherein the signals output by the sensing elements areindicative of respective times of arrival of the photons on the sensingelements, and wherein the method comprises processing the signals inorder to compute an indication of the distance to the corresponding areain the target scene based on the times of arrival.
 13. The methodaccording to claim 12, wherein processing the signals comprises binningtogether the signals that are output by the sensing elements in the setin order to compute an average time of flight of the photons over theset.
 14. The method according to claim 11, wherein the one or more beamsare emitted along a transmit axis, while the target scene is imaged ontothe array along a receive axis, which is offset transversely relative tothe transmit axis, and wherein adjusting the membership compriseschanging the membership so as to compensate for a parallax between thetransmit and receive axes as a function of the distance.
 15. The methodaccording to claim 14, wherein changing the membership comprisesshifting the boundary of each selected region of the array responsivelyto the distance in order to compensate for the parallax.
 16. The methodaccording to claim 15, wherein imaging the target scene comprisessensing the photons received from the target scene, using the array ofsensing elements, over a range of distances from a minimal range to amaximal range, such that a first image cast onto the array of thecorresponding area of the scene at the maximal range is shiftedtransversely, due to the parallax, relative to a second image cast ontothe array of the corresponding area of the scene at the minimal range,and wherein shifting the boundary comprises setting the boundary of theselected region to contain all of the first image but only a part of thesecond image.
 17. The method according to claim 11, wherein imaging thetarget scene comprises sensing the photons received from the targetscene, using the array of sensing elements, over a range of distancesfrom a minimal range to a maximal range, and wherein adjusting themembership comprises setting a size of the one or more selected regionsto be sufficient to contain a first image cast onto the array of thecorresponding area of the scene at the maximal range, but smaller than asecond image cast onto the array of the corresponding area of the sceneat the minimal range.